const mergeTwoLists = (l1, l2) => {
  if (l1 === null) {
    return l2
  }
  if (l2 === null) {
    return l1
  }
  if (l1.val < l2.val) {
    l1.next = mergeTwoLists(l1.next, l2)
    return l1
  } else {
    l2.next = mergeTwoLists(l1, l2.next)
    return l2
  }
};

let l1 = {
  val: 1,
  next: {
    val: 2,
    next: {
      val: 4,
      next: null
    }
  }
}

let l2 = {
  val: 1,
  next: {
    val: 3,
    next: {
      val: 4,
      next: null
    }
  }
}

console.log(mergeTwoLists(l1, l2));